Effect of Temperature on Reaction Rates & the Concept of Activation Energy (Cambridge (CIE) AS Chemistry): Exam Questions

Fig. 1.1 below shows, for a given temperature T, a Boltzmann distribution of the kinetic energy of the molecules of a mixture of two gases that will react together.

The activation energy for the reaction, E_a, is marked.

Fig. 1.1

On Fig. 1.1 above,

i) Sketch a new distribution curve, clearly labelled T₂, for the same mixture of gases at a lower temperature, T₂.

[2]

ii) State the effect of a decrease in temperature on the activation energy, E_a.

[1]

Define the term activation energy, E_a.

i) On Fig. 1.1, draw and label the activation energy for the reaction in the presence of a catalyst, E_a,cat.

[1]

ii) Use the Boltzmann distribution to explain how the addition of a catalyst results in reactions occurring at a faster rate.

[2]

Two reactions involving aqueous sodium hydroxide, NaOH (aq), are given below.

CH₃CHBrCH₃ + NaOH → CH₃CH(OH)CH₃ + NaBr Reaction 1

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O Reaction 2

The reagents in Reaction 1 must be heated together for some time for the reaction to occur. Reaction 2 is almost instantaneous at room temperature.

Suggest why these reactions occur at different rates.

Ammonia can be produced by the reaction of nitrogen and hydrogen in the presence of an iron catalyst.

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g) ΔH = −92 kJ mol^-1

The activation energy for the forward catalysed reaction, E_a,cat, is +109 kJ mol^-1.

Complete the reaction pathway diagram in Fig. 2.1 for the uncatalysed and the catalysed reaction between nitrogen and hydrogen.

You should label the following:

• products

• the enthalpy change of reaction, ΔH

• the activation energy of the forward uncatalysed reaction, E_a

• the activation energy of the forward catalysed reaction, E_a,cat

Fig. 2.1

Calculate the activation energy of the catalysed decomposition of ammonia into nitrogen and hydrogen. Show your working.

activation energy = ......................................................... kJ mol^-1

Catalysts, such as iron used in the production of ammonia, increase the rate of reaction.

Use the Boltzmann distribution to explain how an iron catalyst increases the rate of this reaction.

Platinum is used as a catalyst in catalytic converters, which are fitted to vehicle exhaust systems to remove nitrogen oxide from the exhaust gases.

2NO (g) + 2CO (g) → N₂ (g) + 2CO₂ (g)

i) State whether platinum acts as a homogeneous or heterogeneous catalyst in a catalytic converter.

[1]

ii) Deduce whether nitrogen is oxidised or reduced in this reaction. Explain your reasoning with reference to relevant oxidation numbers.

[2]

In any chemical reaction, the particles will all be moving in different directions, at different speeds and with different amounts of energy. Boltzmann distributions show the distribution of energy amongst particles in a chemical reaction.

Fig. 3.1 below shows the Boltzmann distribution in a sample of a gas at a fixed temperature, T₁.

Fig. 3.1

i) Label the x and y axes of the graph.

[1]

ii) Sketch a distribution for a sample of the same gas at a higher temperature, T₂.

[2]

State why a Boltzmann distribution curve always starts at the origin, and what the area under the curve represents.

Some changes were made individually to the reaction in part (a).

Consider the Boltzmann distribution from Fig. 3.1. For each of the changes in parts (i), (ii) and (iii) below, state and explain the effect that the change would have on:

• The area under the curve

• The value at which the peak of the curve occurs (the most probable energy of the molecules)

• The proportion of molecules with energy greater than or equal to E_a

i) The temperature of the original reaction is increased, but no other changes are made.

[2]

ii) The number of molecules in the original reaction mixture is increased, but no other changes are made.

[2]

iii) A catalyst is added to the original reaction mixture, but no other changes are made.

[2]

A Boltzmann distribution curve is shown below in Fig 1.1

Fig 1.1

For the changes detailed in parts (i) and (ii), state and explain the effect the change would have on:

• The area under the curve

• The value of the most probable energy of the molecules

• The proportion of molecules with energy greater than or equal to E_a

i) The temperature of the original reaction is increased, but no other changes are made.

[2]

ii) A catalyst is added to the original reaction mixture, but no other changes are made.

[2]

The Boltzmann distribution curves for a gas mixture at three different temperatures, 100 K, 300 K and 700 K, are shown in Fig 1.2.

Fig 1.2

i) Label each curve in Fig 1.2 with the correct temperature values, 100 K, 300 K and 700 K.

[1]

ii) A student states that 'All reacting molecules have higher kinetic energy at 700 K than they do at 300 K'.

State whether you agree with the student. Use the Boltzmann distribution to explain your answer.

[3]

Hydrogen reacts with chlorine to form the hydrogen halide, hydrogen chloride, a colourless gas.

H₂ (g) + Cl₂ (g) → 2HCl (g)

i) Give one reason why most collisions between hydrogen and chlorine molecules do not lead to the formation of hydrogen chloride.

[1]

ii) Apart from changing the temperature, state and explain two ways of speeding up the formation of hydrogen chloride.

[4]